It will be based on webkit.

A couple of weeks back I was fortunate enough to meet Rajesh (the guy who made many of these slides) and asked if the move to Qt in Maemo6 would affect their choice of gecko as the microB browser engine.

I was told that they would continue using Firefox as the main browser, and Webkit would be the preferred rendering engine for web apps.

Makes a lot of sense to, gecko is generally better from a compatibility perspective, and webkit is much lighter. Not to mention webkit 'comes for free' with qt 4.4+ already.